Probabilistic Heavy Snow and Icing Discussion NWS Weather Prediction Center College Park MD 410 AM EST Wed Jan 15 2020 Valid 12Z Wed Jan 15 2020 - 12Z Sat Jan 18 2020 ...A trough builds down the West Coast through Thursday before shifting east to the Rockies through Friday and across the Great Plains Friday night... ...West Coast... Days 1-2... Deep low pressure currently well off the OR/WA border will shift northeast today before merging with another low from western Canada tonight off the BC coast. Shortwave energy will dig around this merging low and dive south down the West Coast tonight through Thursday, pushing inland over southern CA Thursday night. An enhanced plume of Pacific moisture will stream inland ahead of this trough with snow levels generally 1000 ft in WA, 2000 ft in OR and far northern CA, 3000ft in northern/central CA, and 4000ft in southern CA. Day 1 probabilities for 8 or more inches are high for the Olympics and highest WA and northern OR Cascades as well as the Klamath Mtns in CA. Day 2 probabilities are high for 12 or more inches in the CA Cascades and down the length of the Sierra Nevada. Only a few inches are expected in the southern CA mountains Thursday night. ...Intermountain West... Days 2/3... The trough will be quite progressive as it ejects inland, moving from the southern CA coast Thursday evening to across the central Rockies by Friday evening. Therefore, snows across the Great Basin are expected to be fairly light, on the order of 4 inches or less. That said moderate Day 2 probabilities for six or more inches across the northern Rockies. A southern stream plume of tropically sourced Pacific moisture spreads in advance of the main trough, providing a plume of precipitation to eastern AZ/and the southern Rockies Thursday/Thursday night. Moderate to high probabilities for 6 or more inches are across the highest terrain in this area (White Mtns of AZ, San Juans of CO/NM and the Sangre de Christos. Snow levels in this plume will be 7000 to 8000ft. The northern stream trough crosses the central Rockies Friday with Day 3 probabilities for 6 or more inches moderate to high for the high terrain of the Wasatch, Tetons, and northern CO Rockies. ...Northern Plains and Great Lakes... Days 1-2... A shortwave trough currently over the Dakotas will quickly shift east to southern New England through tonight. Initially weak surface cyclogenesis will keep precip totals light over MN and the Great Lakes today with lake effect snow trailing the system (aided by a strong surface ridge to the north)...particularly off Lake Superior tonight. Day 1 probabilities for 4 or more inches are moderately high over the Porcupine Mtns of the UP where convergent precip and LES combine. Ahead of the trough, warm air advection will push a warm nose into WI/MI with a stripe of confidently less than a tenth of an inch of ice expected today. Lake effect snow is expected behind the low tonight and Thursday, with the favored NW snow belts receiving the heaviest snow downwind of Lake Superior, and the effective fetch areas off Lake Michigan and Lake Erie. WPC probabilities are moderate for 4 inches of LES. ...New England... Days 1-2... A surface low moving across central late tonight New England will race into the Gulf of Maine while rapidly deepening in response to a negatively tilting mid-level trough and merging jet streaks aloft. Intense ascent in a region of PWATs reaching +1 standard deviation above the mean will produce a period of heavy precipitation before the low exits quickly by late Thursday. Much of central/southern New England will be too warm for snow, but from the Adirondacks, through northern VT/NH and central/eastern Maine, a swath of heavy snow is likely. While the system will be moving quickly, there is an expectation that as the low deepens over New England a TROWAL will wrap cyclonically around the low, aiding ascent provided by a lingering inverted trough. This will enhance snowfall with moderate to high probabilities for 6 or more inches through the swath from the Adirondacks through Maine. ...Great Plains through the Midwest... Days 2-3... The combination of tropical Pacific moisture ahead of an ejecting northern stream trough over the western US and cold preconditioning from a Canadian crossing the Great Plains today to across the Great Lakes through Thursday night, A significant storm system will develop in the lee of the Rockies Friday. As the low begins to develop, intense warm air advection on increasing low-level southerly flow will spread precipitation northward from west Texas Thursday night, spreading up the Great Plains, reaching MN Friday. Broad swaths of mixed precipitation are expected in this plume with freezing rain/sleet, and snow spreading north Friday. All snow is expected north of the surface low track that develops in the lee of the WY/CO Rockies and shifts across the central Plains (likely NE/IA) Friday night. This sets up a heavy snow threat for the eastern Dakotas across MN and into WI. Day 3 snow probabilities are high for 6 inches over much of central MN and the north Lake Superior shore in the Arrowhead where southeasterly flow would be enhanced on the terrain like what has happened in recent days there. A wintry mix over the TX/OK Panhandles Thursday into Thursday night may result in thick ice with Day 2 probabilities there 20 to 40 percent for a quarter inch of ice. This risk area spreads east along the southern side of the developing low with a Day 3 risk area for a quarter inch of ice 20 to 30 percent over much of MO. A wintry mix quickly spreads east along a developing warm front Friday night with Day 3 probabilities for a tenth inch of ice spreading across IL/IN/western OH. A wintry mix between the snow and ice areas (centered on IA) would be mainly sleet, though exact details will be borne out over subsequent forecasts. Jackson